That's basically how federated software has to work. Without defederation, running federated software becomes unusable. Either you get overrun by spammers or you become legally liable for illegal content from other servers if you don't do anything about it (the beehaw admins mentioned someone posting child porn as being one reason for defederation). Lemmy is clearly in its early days but this kind of thing will become way more common, as it is on more mature fediverse platforms.

Email providers are a good example of federated software. They have to make sure nobody is sending spam or malware or they will get federated, and they can be very aggressive about that.

Ultimately if you don't want defederation to ever happen, you want a centralized system run by a single organization. Those are your options.

Or you can have the government step in and have a very highly regulated system like for telephony, where almost nobody gets to run an instance, which seems unlikely in this case.